The present invention relates to thermally reactive powders and a method for their preparation. The present invention further relates to aluminum-coated powders useful in the preparation of intermetallic compounds.
Intermetallic materials, particularly those containing aluminum are known to have good ductility at room temperature, high creep strength, high tensile strength and high resistance to oxidation. The intermetallic phases are usually formed from alloys, however, and it is difficult to form homogeneous materials. In addition, it is difficult to process the alloys into useful materials in the desired final shape.
Thermally reactive powders are used in reactive sintering which is a form of self-propagating high temperature synthesis (SHS). Reactive sintering is accomplished by the formation of a transient liquid phase during the exothermic reaction between the two species of metal powder. The two species are randomly mixed in a near stoichiometric ratio and the mixture is heated under controlled atmosphere, heating rate, time and temperature. At the lowest eutectic temperature in the system, liquid forms and rapidly spreads throughout the structure. The liquid consumes the elemental species and generates an intermetallic species. The interdiffusion of the elements is rapid in the liquid phase and the compound generates heat which accelerates the reaction. In this way it is possible to form a nearly fully dense material.
The close proximity of the two metal species to one another is important in achieving a smooth continuous reaction. One way of obtaining the close contact of the two materials is to coat one with the other.
A conventional method of preparing thermally reactive powders is the direct alloying of metals. In this method a first metal is melted in a heated vessel and a second metal is introduced, whereby a reaction of the two metals takes place, releasing heat and forming a mixture of intermetallic compounds and alloys. The mixture is cooled and the solidified melt is ground into a powder. A significant amount of the starting metal is prereacted during this process which results in a low exothermic effect upon subsequent reaction of the thermally reactive powders.
Metal-coated metals have also been prepared by coating the core metal with an organic binder and adhering the second coating metal onto its surface. However, the coating does not adhere well and impurities (decomposition products of the organic binder) are introduced into the powder during the thermal reaction.
Coating a core metal with a metal salt solution of the second metal followed by thermal decomposition of the metal salt has been used to obtain metal-coated metals. Decomposition of the deposited metal salt results in gas evolution and precipitate formation, thus compromising the quality of the metal coating. Degradation of the metal salt layer in the presence of hydrogen leads to cleaner decomposition products, however, impurities still remain.
It is the object of the present invention to prepare metal-coated metals useful in processes such as reactive sintering. It is a further object of the present invention to prepare metal-coated metal particles that are free from impurities and additives.